15 day heat wave and air conditioning

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Before I start let me just say a few things to establish where I'm
coming from.

I lived in the Adelaide foothills for many years. In summer we used to
get the gully winds (gully gales, really) which would cool the house
overnight. I had a desk fan to stir the air up in the house in the
afternoon. (Btw I bought this desk fan in 1969 at a 2008 equivalent
price of $600 - fans in 2008 are $10-$20.)  The house had wide eaves
and no sun on windows and was generally of good thermal design.

I had refrigerated air conditioning at work. It ran 24 hours/day in
summer and worked well, though one sensed it was wilting towards the
end of a heat wave as all the bricks and concrete of the building
"cooked up". It must have cost many hundreds of dollars per week to
run in a summer heat wave.

I then moved onto the Adelaide plains into a stone and brick house
with concrete floors. Being of heavyweight construction with very
thick walls, the house thermal performance was very good. I added
blinds to keep the sun off windows.

Single hot days were a breeze, and the house remained cool inside.
Heat waves were not so good, and after 4 days the house was getting
hot inside. The house would then take a couple of days to cool down.
30C inside during longer heat waves was common, and I did see inside
temperatures as high as 33C and 36C in exceptional hot weather.

I increased the fans used around the place, since fans were now cheap,
and added window fans to bring in the cool night air. This worked fine
until temperatures overnight would reach 30C, when there was no cool
night air. Luckily this wasn't a frequent occurrence.

The thought of fitting air conditioning was present. However, I didn't
want to spend a bomb on something that wouldn't be used a lot in a
normal year.

Whole house refrigerated air, like at my workplace, was out as far as
I was concerned because of the high expense of both the installation
and operation. I had much better things to spend my money on. :)

This left the alternatives of putting in a split system in my bedroom,
and perhaps another split system elsewhere in the house, or fitting
whole-house evaporative air conditioning.

Thinking about the pros and cons of these alternatives always ended up
with me doing nothing - summer would then end and the problem would go
away for another year. :)

With the split system idea I hated the idea of trying to choose where
I might cool the house. Wherever I cooled would be wrong some of the
time, and the thought of being stuck in a couple of rooms over a
heatwave made me feel stir-crazy just thinking about it.

And even the people who had evaporative air conditioning seemed
apologetic about itů.. (Though Adelaide's low humidity summers were
ideal for evaporative air conditioning.)

Finally I took the plunge and fitted ducted evaporative air to my
house for a whole mish-mash of un-connected reasons. The unit cooled
my whole house, and including the attached garage by opening the
garage access door and letting the air exhaust over the top of the
roller doors.

As you may know we had a record 15 day heatwave in Adelaide recently.
Air conditioners caught fire due to the constant running, and one
house was totally burnt out after the ducting delivered fire
simultaneously to every room. (Domestic air conditioners aren't rated
for constant operation apparently.)

Well, my new evaporative air conditioner performed brilliantly under
these exceptional conditions.

I sometimes ran it 24 hours/day. The house was at 22C every morning at
the start of the day, and it might reach 25C in the afternoon.

The a/c cost about $4/day (cost of water plus electricity) to operate
- a very low cost to cool the whole house plus the garage. An
equivalent refrigerated unit might have cost up to $300/week to run.

I checked how my former work place was going about 10 days into the 15
day heatwave. With my house thermometer showing 25C the temperature in
this workplace "no-expense-spared refrigerative installation" was 24C.

I had a friend who had a household zoned refrigerative air
conditioner. This air conditioner wasn't operated constantly because
of the operating expense. The house internal temperature was 27C.
Outside the zoned area the house was like a furnace

Another friend had an appropriately sized wall unit in a living area.
Again, the house internal temperature was never below 27C after 10
days of heat wave. Outside of the cooled area the heat was intense.

Now, I realize that air conditioners are one of those things that
really divides people. And that for many then evaporative air
conditioners just aren't "real" air conditioners. :) So I don't expect
to change many minds on this.

And yes, I'm aware evaporative air conditioners don't work on the rare
(in Adelaide) high humidity days.

However, my evaporative unit worked brilliantly over a 15-day heat
wave. In fact it worked virtually as well as a no expense spared
refrigerative unit - which no one could afford to run in a domestic
setting - and far better than the usual more economical domestic
refrigerative air conditioners. Apart from still feeling a bit
stir-crazy from being stuck in the house I was comfortable the whole

I'm a convert.


Re: 15 day heat wave and air conditioning
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It's been pretty hot in Melbourne too, and I'm working from home
with ducted evap cooling also. I grew up with it, so I knew it
was what I wanted to install... but I learnt something new this
week. Running the unit on *low* all day can be more effective than
running it on high! The reason being that the higher airflow rate
achieves less temperature drop, I guess. Anyhow, it was definitely
cooler running on low... We probably get more humid days than
Adelaide, but you have to feel hot sometimes, or you forget you're

Re: 15 day heat wave and air conditioning

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I like the both the principle and the reality of evaporative aircon - one
side-effect is that it makes for better-smelling houses as so much fresh air
is flushed through the house. I have friends who work in the dept of the
environment, and when I'm not hassling them about the dubious benefit of
CFLs, I ask them what's the greater evil, the use of water in evaporative
units, or the use of power in refrigerative units. Since almost all of them
are lawyers, not scientists nor engineers, they can't give a reasoned
answer, but in a low-humidity environment, I know which is the better

The catch is, you can't buy an evaporative system at the same local Harvey's
where you bought your plasma TV(s), so marketing, fashion and convenience
govern what people choose.

Re: 15 day heat wave and air conditioning
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I'm in Melbourne, and have had evap cooling for 13 years. It's very
rare that it is so humid that you need to turn off the aircon, maybe
3-5 days per summer. Running costs are indeed low, mostly dominated by
a 600 watt fan motor (flat out). I regard it as air conditioning since
it is still thermodynamically a heat-pump of sorts, using heat of
vapourisation. Something like that, can't remember my uni
thermodynamics much.

Our pommy neighbours got one installed, at first believing that the
summers here are not that hot and they would only use it once a week.
Now they are converts too, and have it on pretty much every day like
us when the temp gets over the high 20's which has been by about 9
oclock on most days since december!

Our unit has a thermostat too. I thought that would not work at first,
but it is brilliant. It has a surprisingly clever control system too,
adjusts motor speed and also will turn off the pump and just use the
fan if it gets cool outside due to a cool change. You can go to bed
and leave it running and eventually it just shuts off.

Re: 15 day heat wave and air conditioning

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My unit has that too. I haven't got to trying the automatic operation
out yet. I guess that will be a treat for next year! :)

I often used the unit just as a big fan to draw in the cool night air.
All the stone, brick and concrete in my house can be used as a
"heat-capacitor" to stabilise house temperature.

I'm going to try using the unit in fan-only mode to heat the house in
winter during the day, again on the "heat-capacitor" principle.

It is great for pushing out cooking smells too. :)


Re: 15 day heat wave and air conditioning

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My solution was a bit more drastic - I left Adelaide 6 years ago for the
Snowy Mountains. The thought of going through another Adelaide summer
brings me out in a cold sweat :)

Re: 15 day heat wave and air conditioning


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$300 per week to run  a domestic installation???

That means 300/168=  $1.80 an hour if it was running continuously.

In Sydney I pay 14.4 cents/kw hour  for top rate electricity
which means the unit would have to use 12.5 kilowatts.
 That is a mighty big ac for domestic use.

By comparison  I have a 19hp unit and a ducted system which uses 6 kw.
It costs me $300 extra  elec bill for the whole summer quarter( and I live
in western Sydney where temps are quite high)
I have the house well insulated and it would only run 50%duty cycle. It can
maintains the temp at 23 if i want but I usually run it at 24.5.
 I think you are exaggerating on the a/c costs.
Evap cooling is pathetic on humid days of which there are a lot in Sydney
especially Feb.

Re: 15 day heat wave and air conditioning

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Isn't 19hp more like 14kW?

Re: 15 day heat wave and air conditioning

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I  am not sure how the air conditioning people calculate the rating of 19
 I think it is something to do with the heat output, taking into account the
efficiency of a heat pump. The energy used by the air conditioner is a
different thing and does not contribute to the heat "pumped"
 The electrical requirement to run the compressor is quite a bit less than
the rated heat output.

 The figures I quoted are taken direct from the manufacturer. 19 hp unit
which uses 6kw.
6kw is also on the plate fixed to the machine.

Re: 15 day heat wave and air conditioning
HI,  take it from me , swampys ( evaps) are useless in humid weather.
evaps are better,  i hate these tosser that have it set at the government 24
c or whatever it is.
that is just plain  retard.
they dont take into account the ammount of people in a building,  and also
what they are doing.they have it cycling the compressor in and out.

 they have it set for a high temp on a hot day, cooling down its ok , but as
soon as the compressor couts out , it starts  warming up  staright  away.,
especially on a  very hot day, you have it set to abvout 18 c,.

mark k
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Re: 15 day heat wave and air conditioning

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Power in Adelaide at summer rates is about 20cents/KWHr.

To provide equivalent performance to a large evaporative unit would
require 20KW cooling power. This would require about 6.0 KW of
electrical power.

Cost/hour = $1.20

Cost/day = $29

Cost week =$203

I copied the original figures from someone else without checking, but
even so it is still pretty high compared to $2 to $4/day, or about 8
cents to 16 cents/hour typical of evaporative units.

I agree a running cost this high is very unlikely to be the case in a
domestic situation, since people don't usually run a/c for a week
straight at 100% duty cycle if only for economic reasons. Nor do they
fit refrigerative a/c units this big.

But then 15 day record heat waves don't happen much either. The power
companies in Adelaide are presently warning people to put aside extra
money for the next power bill.

Adelaide has very low humidity in most hot weather. the winds blow off
the desert from the North and are very dry. It very much suits
evaporative air conditioners. Cooling of outside air by 15C+ is often
achieved.  I don't understand why anyone would install a refrigerative
a/c here.

Obviously they don't suit places like Brisbane or Sydney with the much
higher humidity.

But Perth, Adelaide, Melbourne, Alice Springs etc are very suitable

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Re: 15 day heat wave and air conditioning
I'll hopefully be building a new house soon so I'm also looking into
cooling options.

Is there such a thing as a hybrid where evaporative cooling is used
the majority of the time, with a complementary refrigerative unit
kicking in on humid or really hot days? I'm talking about a ducted
system here.

The other concern is that the upstairs area will be hotter, both
because it's an "attic" style house and I'll also have my room heaters
(currently 5 servers on 24/7) up there. I was thinking of installing a
small split system unit to help remove any additional heat.

If there's such a thing as a hybrid evaporative/refrigerative ducted
system with zoning then I guess that could work well in this case...

Re: 15 day heat wave and air conditioning

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Evaporative and refrigerative are natural enemies - one raises humidity, the
other removes it. Put 2 in the same place and the refrigerative's evaporator
will freeze solid with ice.

I'd suggest going with evaporative, because it will cover you for 95% of the
time and cost you very little to run. But more importantly, if you're
building a new house, make sure it has all the passive features you can
get - extra insulation, thermal mass, wide eves, double glazing, and use
ventilation, forced if required, where you can.


Re: 15 day heat wave and air conditioning

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Funny no one has mentioned what should be the first step IMO. Multiple roof
ventilators (rare considering how cheap they are).
Ceiling fans (cheap to buy and run)
Window awnings (even work on 2 story houses where wide eaves are useless)

BTW too much thermal mass can work against you in a 15 day heat wave, you
need to cool the house as much as possible at night, not retain the heat.
In the "old days" hosing down bricks and roof tiles was a major benefit. If
you have a big enough rainwater tank you can still install a proper spray
(can double as a fire protection system too)

Pity so many cheap passive options seem to be completely ignored in favour
of big refrigerated air-conditioners.


Re: 15 day heat wave and air conditioning
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Would this still happen with a split system unit in the same room as
an evap outlet?

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Definitely. I'm considering some sort of exhaust fan which vents into
the roof to cool the upper floor (drawing cooler air from below) and
push hot air trapped inside the roof space outwards... kind of the
reverse of an evap cooling setup. This would be switched on when the
room temperature downstairs is lower than upstairs, say in the evening
or after a cool change. Because it's an attic house the upstairs area
is basically a box trapped inside a high pitched roof space. It is not
a tiny cramped room (the total area upstairs will be nearly 50sqm / 5
1/2 squares) so it may need some extra attention with regards to

I think the display homes cheat by using refrigerative cooling. :)

Re: 15 day heat wave and air conditioning
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I agree with the others, that would be a bad idea. If you absolutely
must have respite on super-hot days where the evap isn't enough and
you can't just escape to the beach, consider fitting a wall-mounted
refrigerative unit in *just one room*, that you can keep fairly well
closed off from the rest of the house. We have this, but many summers
go by without that getting turned on even for one day. It's a welcome
respite when absolutely needed though, and much cheaper than a full
ducted system.

I think you'll find that a roof-mounted evap unit will cool your whole
house very well. The upper floor needs the most cooling, and gets the
air first, which naturally descends and cools the lower floor before
exiting through open windows. Keep the upper windows shut.

Clifford Heath.

Re: 15 day heat wave and air conditioning
On Sat, 29 Mar 2008 14:04:53 +1100, Clifford Heath

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I agree with Cifford on this.

Installing a one-room refrigerative a/c was my fallback plan if the
evaporative unit fell down on the job enough to bother doing it.

My heavyweight consruction house would carry me through a day or two
of humid weather anyway.

And just having fans improves comfort a lot too.


Re: 15 day heat wave and air conditioning

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Evaporative coolers are quite limited in the amount of cooling they provide.
I doubt if the air would be still cool by the time it got to the ground
If you consult a psychometric chart you can find out the maximum temperature
drop for a given humididy.
 For example, take  Adelaide, which incidentally has an average humidity of
40 to 50 % in the summer months.
On a day where the temperature is 37 degrees C at 50 % humidity the wet bulb
temp is 28 degrees C. so this is the lowest the cooler  can possibly cool
the air. Less for higher humidities.
With an evaporative cooler you must leave your windows open to allow a
constant volume change otherwise they will not work. No recirculation means
that you are constantly cooling hot outside air. Too bad if you have a hot
northerly wind blowing as the wind will come in. Also there will be no heat
insulation provided by shut  glass windows.
I have had experience with them and they are reasonably coolers in still,
dry places like Broken hill and Cobar for example but they are no way
comparable to decent refrigerated air.
Passive solar has been mentioned as the way to go . This is good but your
house will still heat up after a day or too.
The way to go is passive solar AND refrigerated aircon ducted in zones.
I have multiple spinning roof vents, wall and roof insulation and a concrete
slab on ground with 2700 ceilings with ducted air in zones.
The thermostat  only comes on usually about midday . I dont bother to cool
the bedrooms in the day just switch it over for a couple of hours at night
which is enough to cool the rooms
It is possible to fit ceiling vents also which open at night to allow the
hot air to escape through the spinners. One down side of good insulation is
that it keeps the heat in once the house warms up and it cools down outside.
These automatic vents help with this problem.
Refrigerated aircon works,  thats why nearly all commercial premises ,
shopping centres etc use it. Evaporative air can be useful but has limited
applications and in most cases is not as good as refrigerated air. This is
nothing new people have found this out many years ago. In the coastal cities
it is not commom. Go inland and you will see plenty of them up on roofs.
If you have your house properly set up the costs aren't exorbitant for
refrig air and you can take the pain out of summer

Re: 15 day heat wave and air conditioning

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 Humidity in Adelaide falls as temperature rises. Typically humidity
is at 8% to 15% when temperatures are at 37C.

Humidity in Adelaide may be up at 50% (or even more) in the morning
but typically temperatures are 22C to 25C at these humidities.

If it is 30C or 37C early in the morning in Adelaide then the humidity
is also usually low. Even with 30C overnight the humidity is fairly
low overnight and evaporative cooling still works well.

It is easy to get evaporative cooling of 15C+ in Adelaide. Evaporative
coolers will cool to 90% of the difference betwen the wet bulb and dry
bulb temperature.



which gives 9am and 3pm temperatures and humidities in March 2008 at
Edinburgh AFB in the north of adelaide. Other historical
temperature/humidity figures and locations are available.

Generally speaking humidity is quite low when it is hot.

Many large commercial cooling installations in Adelaide use
evaporative pre-coolers too. Often they are two stage coolers to
reduce the humidity in the output air.


Re: 15 day heat wave and air conditioning

The above link says the average rh in summer is 40 to 50  percent.

It is a very sweeping statement to say that the humidity is always low when
the temperature is high.
I find it difficult to believe that there are no humid nights in a coastal

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